This invention relates to a pattern matching system used in speech recognition equipment, and more particularly to two-axis pattern matching on the frequency and time axes using linear predictive coefficients or the like.
This invention also relates to a frequency transformation circuit, more particularly to a circuit for transforming the frequency structure of a signal which can be incorporated in the pattern matching system.
When speaker-independent speech recognition apparatus performs pattern matching, it must first normalize individual differences which are present on both the frequency axis and the time axis. An example of a prior-art system of pattern matching on these two axes is disclosed in the paper "Speaker-Independent Spoken Word Recognition Based on Time-Frequency-Intensity Warping of Spectra", Nakagawa et al, The Transactions of the Inst. of Electronics and Communication Engineers of Japan,'81/2 Vol. J64-D No. 2. This method performs dynamic-programming matching (DPM) on the frequency axis and time axis of discrete output values from bandpass filters.
A problem in the above pattern matching system is that since it uses features obtained from the output of a bank of bandpass filters, it has difficulty in handling the details of frequency patterns accurately. Another method uses the fast Fourier transform (FFT) instead of a filter bank, but this method has the problem of confusing voice pitch effects with vocal-tract effects, and can provide only inexact results. A difficulty associated with speech recognition is that speech signal contains rapid changes along frequency axis.